In recent years, water absorbent resins having high water-absorption performance have been developed, and have been often used mainly for disposable commodities including absorbent articles, such as disposable diapers and sanitary napkins, water retention agents for agriculture and horticulture, industrial water stop agents, and the like. For such water absorbent resins, many monomers and hydrophilic polymers have been proposed as raw materials, and in particular, polyacrylic acid (salt)-based water absorbent resins using acrylic acid and/or salts thereof as monomers have been most widely used for industrial purposes because of their high water-absorption performance. In the case of such polyacrylic acid (salt)-based water absorbent resins, acrylic acid is neutralized before or after polymerization to produce polyacrylic acid salts. Such neutralization and polymerization are disclosed in Patent Documents 1 to 4. In addition, such acrylic acid has been known to contain acrylic acid dimer (Non-patent Literature Document 1).
As water-absorption properties desired for such water absorbent resins as described above, many properties (parameters) have been known, such as centrifuge retention capacity, absorbency against pressure, water-absorption rate, liquid permeability without load, liquid permeability with load, impact resistance, urine resistance, flowability, gel strength, color, and particle size. In addition, many regulations (parameter measurement methods) have been proposed for one physical property (e.g., centrifuge retention capacity) from various points of view.
Water absorbent resins have been developed with a focus on these many physical properties. However, there has been a problem that even if the above many physical properties (e.g., “centrifuge retention capacity”, “absorbency against pressure”) are controlled, the performance of water absorbent resins when they are put to practical use in absorbers such as disposable diapers remains less than satisfactory. Further, since water absorbent resins are used mainly for hygiene materials such as disposable diapers and sanitary napkins, water absorbent resins are required to be white at the time of shipment from factory, so as not to provide sense of foreign matter due to coloring when powdery water absorbent resins are combined with white pulp in hygiene materials. Further, although water absorbent resins are generally white powder, they are known to be colored (colored yellow to brown) over time after shipment, such as during storage and transportation, and after they are used in hygiene materials, and it has been desired that even when absorbent articles are stored for a long period of time, water absorbent resins of the absorbent articles remain white. In recent years, the proportion (% by mass) of water absorbent resins used in hygiene materials has been increasing, and therefore, the problem of coloring has been becoming more important.
In response, various proposals for improvement of whiteness and prevention of coloring in water absorbent resins are disclosed in, for example, Patent Documents 5 to 31. More specifically, the following are known as methods for controlling a polymerization inhibitor in a monomer: a technique in which the amounts of methoxyphenols in acrylic acid are set to be 10 to 160 ppm (Patent Document 5); a technique in which the amount of hydroquinone in acrylic acid is controlled to be 0.2 ppm or smaller (Patent Document 6); a technique in which a monomer is treated with activated carbon (Patent Document 7); a technique in which tocophenol is used as an inhibitor (Patent Document 8); a technique in which an N-oxyl compound, a manganese compound, or the like is used as a polymerization inhibitor (Patent Document 9); and a technique in which methoxyphenol and a specific polyvalent metal salt are used (Patent Documents 10 and 11).
Further, the addition of anti-coloring agents for water absorbent resins has been known, of which examples may include: the addition of a reducing agent such as a hypophosphorous acid salt (Patent Document 12); the addition of an antioxidant agent (Patent Documents 13 and 14); the addition of a metal chelator and optionally another reducing agent (Patent Documents 15 to 19); and the addition of an organic carboxylic acid and optionally another compound (Patent Documents 20 to 23). Further, Patent Documents 24 to 26 have been known as techniques focusing on a polymerization initiator. Further, there have also been proposed techniques focusing on the amount of iron, as a substance responsible for coloring, in aluminum or a reducing agent (Patent Documents 27 and 28). There has also been known a technique in which ammonium acrylate is used for a monomer (Patent Document 29). Still further, there has also be known the control of the amount of oxygen at a drying step or a surface-crosslinking step (Patent Documents 30 and 31).
However, even with the coloring prevention methods in Patent Documents 5 to 31, variations occur in whiteness of particulate water absorbent resins obtained by continuous polymerization. Particulate water absorbent resins are strongly demanded to be white, and therefore, there is still room for improvement in whiteness. In addition, according to conventional coloring prevention techniques in which the purity of a raw material for particulate water absorbent resins, such as acrylic acid, is highly increased, or the polymerization or drying conditions of water absorbent resins are made mild, or a new anti-coloring agent (e.g., a reducing agent) is used, there is a possibility that the production cost is increased, the productivity is reduced, or the safety properties and water-absorption properties are deteriorated due to the use of an anti-coloring agent.
In addition to the problem of coloring disclosed in Patent Documents 5 to 31, there has been known a problem of the odors of water absorbent resins themselves. Water absorbent resins are required to have no abnormal odor even before being used, so as not to provide a sense of unpleasantness to users of hygiene materials such as disposable diapers or sanitary napkins in which the water absorbent resins are used. In recent years, with an improvement in the performance of water absorbent resins, there have been cases where an unpleasant odor or an abnormal odor, which is considered to be derived from a newly-used substance, has become a problem.
In response, various methods of reducing an odor have been proposed. More specifically, it has been proposed to reduce the odors of the following substances: an acrylic acid oligomer (Patent Document 32); acetic acid and propionic acid (Patent Document 33); a volatile organic solvent (Patent Document 34); a sulfur reducing agent (Patent Document 35); and an alcoholic volatile substance (Patent Document 36). However, these methods are less than satisfactory in that excellent anti-yellowing performance and excellent absorption capability cannot be exhibited. In particular, when a reducing agent is used and high physical properties are achieved, and especially when an absorbency against pressure (AAP) of not lower than 20 [g/g] or a saline flow conductivity (SFC) of not lower than 30[×10−7·cm3·s·g−1] is achieved, an abnormal odor may occur from unknown origin other than the reducing agent.
As described above, it cannot be said that the conventional techniques have sufficiently solved the problem of coloring and other problems (e.g., those of cost, water-absorption performance, and odor). In particular, it is quite difficult to solve the problem of coloring while achieving high water-absorption performance. According to the conventional techniques, coloring is likely to occur especially in the case of a water absorbent resin having an excellent balance between centrifuge retention capacity (CRC) and saline flow conductivity (SFC), for example, in the case of a water absorbent resin having a CRC of not lower than 25 [g/g] and an SFC of not lower than 50[×10−7·cm3·s·g−1], while the balance between the CRC and the SFC may greatly be deteriorated even by the addition of an anti-coloring agent in a small amount.
For example, there is a case where an aluminum compound is used as a liquid permeability improver. As described above, however, an aluminum compound is known to contain iron which is a causal substance for coloring (Patent Document 27). A water absorbent resin to which an aluminum compound is added is likely to be colored. If it is attempted to reduce the coloring by using an anti-coloring agent as described above (Patent Documents 12 to 23), a large amount of the anti-coloring agent may be needed, resulting in a deterioration of the balance between the CRC and the SFC.